1. Field of the Invention
The present invention relates to a transformer having a simple structure with a high productivity, and more particularly, to a transformer and a power supply for cancelling out conducted noise or conducted noise and radiated noise due to a capacitive coupling between transformer windings to provide a small EMI deviation and a sufficient margin even during mass production, thereby reducing the unit cost of the transformer, and reducing the cost of the EMI filter.
2. Description of the Related Art
So far, there has been a magnetic energy-transfer element or power supply configured to reduce a displacement current flowing to the electrical ground from the power supply using a cancellation effect due to magnetic energy-transfer element windings. However, five or six strands of thin wire should be stretched and wound to fill one winding layer with no gap using a small number of turns for cancellation. Accordingly, it has may disadvantages in that the winding operation is difficult and the productivity of the transformer is low, thus increasing the unit cost, and in case of a transformer with a low profile form factor, several strands cannot be connected to a pin, and the like. Furthermore, modified shaped methods have been used to get out of the restriction of the height of a transformer having a low profile form factor. In this case, it has a large deviation in the effect of reducing a displacement current flowing to the electrical ground from the power supply, thus causing a disadvantage in that it is difficult to satisfy EMI standards.
The prior art will be described below in brief.
FIG. 1 is a view illustrating a principle in which the transformer 13, input line 16 and output line 17 are coupled by a distributed capacitance within the transformer in a typical flyback converter to generate a displacement current to the electrical ground. Hereinafter, a black dot shown for each winding of the transformer indicates the start or end of the winding.
Referring to FIG. 1, an AC input voltage is rectified and smoothened by the capacitor 11. The switching element 12 is switched in response to the feedback of the output voltage to create the storage and transferring of energy in the input winding 131 of the transformer 13, and the output rectifier 14 and capacitor 15 rectifies the voltage of the output winding 133 to supply power to a load.
Typically, the varying speed of voltage at the connection point between an end of the input winding 131 of the transformer 13 and the switching element 12 is very fast when the switching element 12 is turned on or off, and the potential variation of maximum 500-600 volts occurs. The potential variation is transferred to the output winding 133 through the path of a distributed capacitance (Cps) between the input winding 131 and the output winding 133 or the path of a distributed capacitance (Cpc) between the input winding 131 and the transformer core and a distributed capacitance (Csc) between the transformer core and the output winding 133, thus allowing the output line 17 to have a noise potential. The potential variation allows the input line 16 to have a noise potential through a distributed capacitance (Cpi) between the input winding 131 and the input line 16. Furthermore, the potential variation allows the transformer core to have a noise potential through a distributed capacitance (Cpc) between the input winding 131 and the transformer core 136. Those noise potentials allow a current to flow through a distributed capacitance (Cig) between the input line 16 and the ground, a distributed capacitance (Cog) between the output line 17 and the ground, and a distributed capacitance (Ccg) between the transformer core and the ground, thereby generating common mode noise, and thus the noise current should be managed to be less than a level specified by the regulations.
FIG. 2 is a principle view for cancelling a capacitive coupling of the output winding by a potential of the input winding in the related art.
Referring to FIG. 2, the input winding 131 generates a capacitive coupling current through a distributed capacitance of the surface facing the output winding 133 by generating an electric field in the direction of facing the output winding 133, and generates a capacitive coupling current through a distributed capacitance between the input winding 131 and the transformer core 136 and a distributed capacitance between the transformer core 136 and the output winding 133 by generating an electric field in the direction opposite to that of facing the output winding 133.
Referring to FIG. 2, a capacitive coupling current between the input winding 131 and the output winding 133 should be maintained low to maintain a displacement current flowing to the electrical ground through the output line. In FIG. 2, to this end, an electric field generated in the direction of facing the output winding 133 from the input winding 131 is shielded by the cancellation winding 132, and an electric field generated in the direction opposite to that of facing the output winding 133 from the input winding 131 is shielded by the shield winding 134.
Furthermore, a capacitive coupling generated in spite of the shielding is removed by the shield winding 134 that forms an electric field using a potential having a polarity opposite to that of the input winding 131. Furthermore, the cancellation winding 132 generates a capacitive coupling having a reversed polarity between the cancellation winding 132 and the output winding 133, thereby cancelling out a capacitive coupling between the input winding 131 and the output winding 133 generated in spite of the shielding.
In order to generate a current having a reverse polarity for cancelling out a capacitive coupling generated from the input winding 131 having a high potential variation to the output winding 133 having a low potential variation with the same polarity, the cancellation winding 132 should have a potential variation lower than that of the output winding 133, and to this end, the number of turns (T: turn) of the cancellation winding 132 is less than that of the output winding 133.
For example, a transformer having a winding width of 8 mm, which is widely used for a mobile phone charger power supply with the input of a commercial voltage of 220 V and the output of 5 V is taken as an example. When the number of turns of the output winding 133 is 8T (T: turn), the number of turns of the cancellation winding 132 is 6T to 7T to cancel out the coupling while shielding the input winding 131 from being capacitively coupled to the output winding 133. In order to completely surround the winding width of 8 mm with 7T, six strands of thin wire having a diameter of 0.18 mm should be uniformly stretched and wound in parallel with no gap, and thus the winding work may be difficult, thereby reducing the productivity and increasing the cost.
FIG. 3 illustrates an example of the transformer of FIG. 2, and FIG. 4 is an example further including three strands of bias winding 135 for pulling out an auxiliary power of about 10 V from the transformer of FIG. 3. Total nine strands should be connected to a common grounding terminal (5a and 7a) to which three strands of bias winding 135 and six strands of cancellation winding 132 should be connected, but such a method cannot be used for a small-sized product in which the height of soldered components is restricted.
FIG. 5 illustrates the structure of a modified transformer for enhancing the productivity of a winding. It has a structure in which the bias winding 135 having a number of turns far greater than that for cancellation is located between the input winding 131 and the output winding 133, and one strand of cancellation winding 137 capacitively coupled to part of the output winding 133 to cancel out a capacitive coupling generated between the input winding 131 and the output winding 133 and between the bias winding 135 and the output winding 133 is added. However, a barrier tape 138 for holding the location of the cancellation winding 137 has a large width deviation, and the physical location of the cancellation winding 137 is varied, and thus a large deviation occurs at a coupling between the cancellation winding 137 and the output winding 133. The deviation has a disadvantage in that EMI is generated to a large extent according to the product.
FIG. 6 is an example having a sandwich winding structure in the related art, in which it is divided into a first input winding 131a having a small potential variation width and a second input winding 131b having a large potential variation width of the input winding to surround both the winding surfaces of the output winding 133 in a sandwich shape. The first shield winding 132a and second shield winding 132b are located between the first input winding 131a and the output winding 133 and between the second input winding 131b and the output winding 133, respectively, to shield a capacitive coupling between the first input winding 131a and the output winding 133 and between the second input winding 131b and the output winding 133. However, even though a capacitive coupling between the second input winding 131b and the output winding 133 having a large potential variation width is shielded, it generates a coupling current far greater than the coupling current occurring between a winding layer having the lowest potential variation width among the winding layers of the input winding 131 in FIG. 3 and the output winding 133. Furthermore, a high spike voltage inherent in the second input winding 131b having a large potential variation width forms another noise on the second shield winding 132b. Accordingly, large conducted noise and radiated noise may be generated, thus requiring measures for noise reduction such as reinforcing line filters, using high frequency filters, and the like.
According to the related art, six strands should be wound in parallel for a wire, and thus the automation is difficult and the productivity is reduced, and soldering a large number of wires to terminals does not satisfy the height restriction in small-sized products, and the shielding deviation is high when shielding with bias windings and balanced windings to reduce the number of strands of wire, thereby deteriorating the EMI margin management. Furthermore, large conducted noise and large radiated noise may be generated in a sandwich winding structure, thus having disadvantages of requiring measures for noise reduction such as reinforcing line filters, using high frequency filters, and the like. The present invention is contrived to solve all the foregoing disadvantages in the related art.